The X-ray CT apparatus is an apparatus that acquires a tomographic image (hereinafter, described as a reconstructed image) of an object by calculating an X-ray absorption coefficient from an X-ray transmission image (hereinafter, described as projection data) of the object imaged from a plurality of directions. The X-ray CT apparatus is widely used in the medical or non-destructive inspection field. In particular, increasing the speed of rotational driving or arranging X-ray detectors in multiple stages in a rotation axis direction is in progress in the medical field in recent years. In this manner, it has been possible to image the whole of moving organs, such as the heart, without causing blurring.
An X-ray tube used as an X-ray source in these X-ray CT apparatuses generates X-rays by accelerating thermal electrons, which are generated by the filament, with a high voltage while making the thermal electrons converge on the focus and collide with the rotating anode target. In this case, some of the energy of the thermal electrons is converted into X-rays, but most of the energy of the thermal electrons is converted into heat. Accordingly, the temperature at the focus becomes high. Due to this heat, the temperature of the rotary shaft that supports the X-ray target is increased and this causes expansion and contraction (hereinafter, referred to as thermal expansion). As a result, the focus position changes. Then, since the generated heat is directed to the outside by radiation or a cooler, the temperature of the rotary shaft of the X-ray target or the like falls. Since this causes a contraction, the focus position changes again. In many X-ray CT apparatuses, as shown in FIG. 19, a rotary shaft 402 of an X-ray target 400 in an X-ray tube 100 is disposed such that the direction of the rotary shaft 402 matches the direction of a rotary shaft 403 of a gantry rotation unit 101, and this direction matches a slice direction 107 of an X-ray detector 104. Therefore, if the focus position shifts due to the thermal expansion of the rotary shaft 402 of the X-ray target or the like occur, the X-ray irradiation range changes in the slice direction 107.
Such thermal expansion may cause the degradation of image quality, such as the occurrence of artifacts or a lowering in the quantitative capability, in a reconstructed image. This phenomenon will be described with reference to FIG. 20. FIG. 20 is an explanatory diagram showing that a change in the X-ray irradiation range becomes the cause of the occurrence of artifacts, a lowering in the quantitative capability, and the like, where FIG. 20(a) shows an example of the X-ray irradiation range and FIG. 20(b) shows an example of the X-ray irradiation range that is different from FIG. 20(a). FIG. 20 shows two X-ray detection elements 228, which are located at the end in a slice direction and are adjacent to each other in a channel direction 108, in an X-ray detector in which the X-ray detect ion elements 228 are arranged in a two-dimensional manner in the slice direction 107 and the channel direction 108. In addition, in FIGS. 20(a) and 20(b), X-ray irradiation ranges 404 are different.
There is a positional deviation in the slice direction 107 between X-ray detection elements 228-1 and 228-2 described in FIG. 20. This is because of positional deviation or deformation occurring in scintillator elements or photodiode elements which form the X-ray detection elements at the time of manufacturing or assembly, positional deviation or deformation occurring at the time of bonding or mounting of a block substrate configured to include scintillator elements or photodiode elements, positional deviation between arranged modules when an X-ray detector is formed by a plurality of X-ray detection modules, or the like.
It is difficult to completely eliminate such positional deviation.
Thus, when there is a positional deviation in the slice direction between the X-ray detection elements 228, the X-ray detection elements 228-1 and 228-2 show different changes if the irradiation range moves in the slice direction 107 from FIG. 20(a) to FIG. 20(b). In the X-ray detection element 228-1, the output does not change since X-rays are incident on the entire X-ray detection element in both the cases shown in FIGS. 20(a) and 20(b). On the other hand, in the X-ray detection element 228-2, in the case shown in FIG. 20(b), the output is reduced since X-rays do not strike apart of the X-ray detection element. Such an output change that differs depending on the X-ray detection element 228 causes the occurrence of artifacts or a lowering in the quantitative capability in a reconstructed image.
In order to prevent a change in the X-ray irradiation range due to such a focus shift, for example, as disclosed in PTL 1, an X-ray collimator is moved by estimating the focus position at the time of next X-ray irradiation using the focus position detected at the time of the last X irradiation and cooling characteristic data.